feat: implement full admin panel with dashboard, user management, and platform settings

Adds complete super_admin panel with 9 pages and account owner categories page.
Backend includes 5 new DB tables, ~25 API endpoints, settings manager with
in-memory cache, and 29 integration tests. Frontend includes reusable admin
components (DataTable, Pagination, ActionMenu, etc.) with code-split lazy loading.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/models/__init__.py

@@ -16,6 +16,9 @@ from .step_library import StepLibrary, StepRating, StepUsageLog
 from .refresh_token import RefreshToken
 from .audit_log import AuditLog
 from .session_share import SessionShare, SessionShareView
+from .account_limit_override import AccountLimitOverride
+from .feature_flag import FeatureFlag, PlanFeatureDefault, AccountFeatureOverride
+from .platform_setting import PlatformSetting
 
 __all__ = [
     "User",
@@ -41,4 +44,9 @@ __all__ = [
     "AuditLog",
     "SessionShare",
     "SessionShareView",
+    "AccountLimitOverride",
+    "FeatureFlag",
+    "PlanFeatureDefault",
+    "AccountFeatureOverride",
+    "PlatformSetting",
 ]